1. Field of the Invention
The present invention relates to an organic EL display, which is one kind of flat-panel displays.
2. Description of the Related Art
As flat-panel displays, liquid crystal displays and plasma displays are commercialized. However, liquid crystal displays have such problems as a narrow viewing angle and insufficient response to high-speed pixel signals, and plasma displays have such problems as a large amount of power consumption and many technical subjects to be solved for increasing to a larger size than current one.
What is drawing attention recently in contrast to these is an organic electroluminescence display (organic EL display) that uses organic light-emitting materials. By using organic compounds as light-emitting materials, it is expected as a display that can realize a flat-panel display that is self light-emitting, high in response speed, and further free from viewing angle dependency with low power consumption.
Organic EL displays include a simple matrix type one that does not use a thin film transistor (TFT), and an active matrix type one that uses TFT. The simple matrix type display is composed of parallel plural electrodes, an organic EL layer, and plural electrodes perpendicular to the parallel plural electrodes, which are formed on a glass substrate. In a simple matrix type display, the organic EL layer is required to perform very high-brightness emission in a selected moment, thus there are such problems that the organic EL layer degrades severely and wiring resistance results in large voltage drop. On the other hand, in an active matrix type display, on a glass substrate, a TFT circuit is formed, on which an organic EL layer exists, and further a counter electrode is formed. Usually, for a semiconductor of the TFT circuit, amorphous silicon or polysilicon is used. In this case, generally, the counter electrode is used as a cathode to take out light from the substrate side.
However, in active matrix type organic EL displays based on selective coating of R, G and B light-emitting layers, there is such a problem that color purity is insufficient, that is, green is whitish and red is yellowish. In addition, there is such a problem that a “white EL+color filter” system and a “blue EL+color conversion layer” system can be applied only to simple matrix type displays. For the purpose of solving these problems, there is such a method that color filters are formed on TFT on a substrate, on which an organic EL layer is formed (JP-A-2001-249627, JP-A-2002-184575, JP-A-2004-207065, JP-A-2004-303609). However, in the method, it is necessary to connect the TFT and the organic EL by means of a via hole, to result in a complex construction and low reliability.
In addition, since the amount of light that can be taken out from the substrate side is limited to that from a portion given by subtracting the area of nontransparent TFT, an aperture ratio (=light-emitting area/pixel area) is small, and then light with high brightness must be emitted (although not so small for simple matrix types).